This invention relates to hydraulic percussive machines such as rock drills.
In such machines it has already been proposed to dampen the stroke of the piston under no-load conditions by providing a dashpot into which a flange on the piston moves on the working stroke. As a result of the dampening action heat is generated in the fluid trapped in the dashpot and unless that heat be dissipated overheating can occur after repeated no-load conditions, e.g. with hand-held machines operated by inexperienced operators or operated in difficult ground.
The dashpot is usually at the forward end of a chamber at the front end of the drilling machine. As it were the dashpot forms a dead end to that chamber. In one such prior proposal (see U.S. Pat. No. 4,020,747) it has been proposed to have the chamber in series with the valve controlling the working stroke so that on the working stroke fluid flows through the chamber and to the valve. Eventually the fluid thus flowing goes to tank. However, since the dashpot is a dead end to the chamber and there is little or no agitation of the fluid in the dashpot, not much of the overheated fluid in the dashpot gets into circulation so that after prolonged no-load working, overheating is bound to occur.